What's the difference between heartburn and acid reflux?

Sometimes after you eat, you might experience a painful, burning sensation in your chest. It feels like someone has run a knife through you just beneath the sternum and is taking their time twisting it around.

Although it can feel like your heart is being squeezed in the palm of a giant, what you're feeling is actually the result of what happens when contents of the stomach -- recently swallowed foods and liquids, bile and stomach acid -- climb up the esophagus.

When food enters your mouth, digestion begins. Saliva begins to break down the starch contained in your food into smaller molecules. Food is then carried down the esophagus into the stomach, where glands in the lining of the stomach create more digestive products, one of which is stomach acid.

The esophagus is a long tube (about 10 inches (25.4 centimeters) long for adults) that connects your throat to your stomach. When you swallow food, you start a wavelike motion in the muscles that line the esophagus, and this motion carries food down toward your stomach. When food reaches the end of your esophagus, it must pass through a ring of muscle -- the lower esophageal sphincter (LES) -- in order to reach the stomach. When objects approach the LES from above, this valve opens inward to allow entry into the stomach. Once the objects have passed through the valve, the valve closes, and pressure exerted on the valve from the stomach only further seals the one-way valve. However, not all valves function perfectly all the time (or, in some cases, at all).

Sometimes, due to a malfunctioning LES, acid reflux and heartburn occur. How does this happen? And aren't acid reflux and heartburn the same thing? (No.)